The occurrence of episodic acidification in Canadian streams, lake waters and shallow groundwaters has been reviewed, and the controlling mechanisms identified. 'Episodes', which are periods of depressed alkalinity during hydrological events, have been studied mainly in southeastern Canada, and occur at all sites where there is sufficient time resolution of the observations, viz. Ontario, Quebec and Nova Scotia. An 'alkaline episode', where acidity decreases during an event, has been reported from one lake in the Canadian Arctic. There is a bias towards the examination of episodes stimulated by snowmelt or rain-on-snow, since rainfall-stimulated episodes are poorly documented. Pre-event, rather than event, water dominates runoff during episodes. For this reason, biogeochemical reactions and the hydrological flowpaths in operation through the vadose and saturated zones are the principal controls on the chemical characteristics of episodes. (Copyright (c) 1994 Kluwer Academic Publishers.)